Alumina based ceramic material and method of manufacturing the same

Abstract: A mixed powder containing: 5-30 wt% of SiC whiskers; 3-30 wt% of a sintering auxiliary agent composed of one kind or more of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements; and an Al₂O₃ powder as a base material; or further containing 0.5-40 wt% of one kind or more of compounds composed of one kind or more of transient elements in the groups IVa, Va, and VIa of the periodic table and one kind or more of C, N and B, are compacted and subjected to pressureless-sintering in a nitrogen containing inert gas atmosphere at a temperature between 1500 and 1900°C. With this method, the SiC Whiskers containing Al₂O₃ based ceramic material, which is densified and has a sufficient strength and toughness, can be formed in a complex shape and a large size.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an Al 2 O 3 (alumina) based ceramic material and a method of manufacturing the same. In particular, the present invention concerns an Al 2 O 3 based ceramic material suitable for tools requiring high strength, toughness and wear resistance, such as cutting tools and a die extrusion plug, and mechanical parts requiring wear resistance, toughness and shock resistance, such as a pump valve and a mechanical seal; and a method of manufacturing the same. 2. Description of the Related Art An Al 2 O 3 based ceramic material, having a high strength and being excellent in corrosion resistance, oxidization resistance and wear resistance, has been known to be suitable for mechanical parts and jigs/tools. However, it is inferior in strength and toughness to a Si 3 N 4 based ceramic material which has been extensively used as an engineering ceramic material. Various attempts have been made to improve the strength and toughness of an Al 2 O 3 based ceramic material. One is to add SiC whiskers to an Al 2 O 3 based ceramic material, as disclosed in U.S. Patent No. 4,543,345. This method is successful to obtain the excellent toughness, however, it presents a problem in the manufacturing process. Namely, in this method, an Al 2 O 3 based ceramic material added with SiC whiskers (hereinafter, referred to as "SiC whiskers containing Al 2 O3 based ceramic material") is manufactured by hot-pressing, and is difficult to be formed into a complex shape or into a large size. JP1103267 discloses a ceramic composite material obtained by evenly dispersing a SiC whisker in an amount of 5-50 vol% into an alumina matrix (50-95%). The alumina matrix may contain at least one of TiB2 and ZrB2 in an amount of 10-50 vol%. The ceramic composite material may also comprise a small amount (0.5 wt%) of MgO. A slurry is prepared which is then subjected to a granulating process to produce alumina - SiC whisker composite granules. The granules are then pressed and sintered at 1800°C under a high pressure to obtain the composite ceramic body. WO91/08994 discloses a fibre-reinforced ceramic composite body based on Al 2 O 3 which is made by a combined sintering and a hot-isostatic pressuring process performed in one heating stage without intermediate cooling. The mechanical properties are improved by the addition of 0.05 to 2% mass of one or more noble metals. To solve this problem, a pressureless-sintering process has been attempted for a SiC whiskers containing Al 2 O 3 based ceramic material. For example, Terry N. Tigers et al, disclose a method in J. Am. Ceram. Soc., 73[5] (1990) 1440-1442 wherein the pressureless-sintering is performed while the aspect of SiC whiskers and the kinds of sintering auxiliary agents are changed. Young-Wook Kim et al, propose a method in J. Mat. Sci., 26 (1991) 1316-1320 wherein the pressureless-sintering is performed using a special process of precipitating aluminum hydroxide in a green compact before sintering. On the other hand, the present inventors disclose a method in Unexamined Japanese Patent Publication No. HEI 4-124058, wherein Al 2 O 3 is added with SiC whiskers and a sintering auxiliary agent composed of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements. In addition, either of these pressureless-sintering methods is performed at a temperature between 1500 and 1900°C in an inert gas atmosphere such as argon for preventing the reaction between SiC whiskers and the atmospheric gas. WO 93/11086 discloses a pressureless sintering process for making whisker-reinforced aluminum composites using about 2 wt% to about 7.5 wt% of a nitride modifier consisting essentially of a silicon nitride, aluminum nitride or mixtures thereof to produce a sintered body having a density of greater than about 95% theoretical. The above-described prior art methods using pressureless-sintering, however, have a disadvantage in that a sufficiently densified sintered body is very difficult to obtain because the sintering is obstructed by SiC whiskers. To cope with this disadvantage, there may be considered methods of, for example, reducing the added amount of SiC whiskers to the extent that a sufficiently densified sintered body can be obtained without any obstruction of the sintering caused by SiC whiskers, and substantially reducing the aspect ratio of SiC whiskers or increasing the added amount of a sintering auxiliary agent as described in J. Am. Ceram. Soc. 73[5] (1990) 1440-1442 . However, these methods tend to significantly lower the strength and toughness of an Al 2 O 3 sintered body. Accordingly, a technique is required which is capable of manufacturing a SiC whiskers containing Al 2 O 3 based ceramic material composed of a sintered body being sufficiently densified and having a sufficient strength and toughness, by pressureless-sintering. In addition, the manufacturing method by hot pressing described in U.S. Patent No. 4,543,345 can manufacture only a product having a simple shape and a small size, and furthermore, it takes a lot of labor. SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems of the prior art, and to provide a SiC whiskers containing Al 2 O 3 based ceramic material composed of a sintered body being sufficiently densified and having a sufficient strength and toughness, basically by not using hot-pressing but by using pressureless-sintering capable of forming a product having a complex shape and a large size. Another object of the present invention is to provide a method of manufacturing a SiC containing Al 2 O 3 based ceramic material composed of a sintered body being sufficiently densified and having a sufficient strength and toughness, basically using pressureless-sintering. According to a first embodiment the present invention provides an Al 2 O 3 ceramic material comprising an Al 2 O 3 sintered body, said Al 2 O 3 sintered body comprising: 5-30 wt% of SiC whiskers, 3-14 wt% of a sintering auxiliary agent selected from one or more oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements, and 0.2 wt% or more of nitrogen, obtainable by a method comprising the steps of: compacting a mixed powder comprising: 5-30 wt% of SiC whiskers, and 3-14 wt% of said sintering auxiliary agent, the balance being an Al 2 O 3 ceramic raw powder, and pressureless-sintering the green compact at a temperature between 1500 and 1900°C in a nitrogen containing inert gas atmosphere. According to a second embodiment the Al 2 O 3 sintered body may further comprise: 0.5-10 wt% of at least one carbide, nitride or boride of transition element selected from groups IVa, Va, and VIa of the periodic table and 0.2 wt% or more of nitrogen, excluding any nitride of said transition element. According to a third embodiment the Al 2 O 3 sintered body of the present invention preferably has a porosity of 1% or less. According to a fourth embodiment of the present invention, there is provided a method of manufacturing an Al 2 O 3 ceramic material according to the invention comprising the steps of: compacting a mixed powder containing: 5-30 wt% of SiC whiskers, and 3-14 wt% of a sintering auxiliary agent selected from one or more oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements, optionally, 0.5 to 10 wt% of at least one carbide, nitride or boride of a transition element selected from groups IVa, Va and VIa of the periodic table, the balance being Al 2 O 3 ceramic raw powder; and pressureless-sintering the green compact at a temperature between 1500 and 1900°C in a nitrogen containing inert gas atmosphere. The method may further comprise the step of hot-isostatic-pressing the sintered body at a temperature between 1200 to 1700°C in a nitrogen containing inert gas atmosphere, thereby reducing the porosity of the sintered body to 1% or less. The above method of manufacturing an Al 2 O 3 ceramic material preferably further comprises the step of hot-isostatic-pressing the sintered body obtained by the pressureless-sintering, at a temperature between 1200 and 1700°C in a nitrogen containing inert gas atmosphere, thereby reducing the porosity of the sintered body to be 1% or less. In the above Al 2 O 3 based ceramic material, the sintering auxiliary agent preferably contains 0.2-10 wt% of MgO, 0.2-20 wt% of ZrO 2 and 1.0-20 wt% of Y 2 O 3 such that the total content of these oxides is 20 wt% or less. In the above Al 2 O 3 ceramic material, the particle size of Al 2 O 3 in the Al 2 O 3 sintered body is preferably 5 µm or less. In the above Al 2 O 3 based ceramic material, preferably, the average aspect ratio of the SiC whiskers is 3 or more, and the diameters of the SiC whiskers are in the range of from 0.4 to 1.5 µm. In the above Al 2 O 3 based ceramic material, the product of the average aspect ratio of the SiC whiskers and the content of the SiC whiskers in the Al 2 O 3 sintered body is preferably 400 or less. In the above method of manufacturing an Al 2 O 3 based ceramic material, preferably, the mixing for obtaining the mixed powder is performed as set forth in claim 11. In the above method of manufacturing an Al 2 O 3 based ceramic material, preferably, the mixing for obtaining the mixed powder is performed by a wet mixing method, and the drying for the mixed powder after the mixing and before the compacting is performed by a spray-drying method. In the above method of manufacturing an Al 2 O 3 based ceramic material, the compacting pressure for the mixed powder is preferably in the range of from 294 MPa (3.0 ton/cm 2 ) or more. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the compacting pressure upon die-pressing a mixed powder and relative densities of a green compact, a sintered body and an HIP material, according to Example 8; and Fig. 2 is a graph showing the relationship between the compacting pressure upon cold-isostatic pressing a mixed powder, and relative densities of a green compact, a sintered body and an HIP material, according to Example 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have studied a technique obtaining a densified sintered body having a sufficient strength and toughness, basically using pressureless-sintering. In the course of this study, it was found that in pressureless-sintering a mixed powder containing SiC whiskers, a sintering auxiliary agent composed of one kind or more of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements (hereinafter, referred to as oxides of Mg and others) and an Al 2 O 3 based ceramic raw powder, a sintering atmosphere exerts an effect on the densification of an Al 2 O 3 sintered body. Specifically, in the case where sintering is performed in a nitrogen containing inert gas atmosphere, the above sintering auxiliary agent effectively acts to obtain an Al 2 O 3 sintered body being sufficiently densified and having a sufficient strength and toughness. The present invention has been accomplished mainly on the basis of this knowledge. A mixed powder containing SiC whiskers, a sintering auxiliary agent and an Al 2 O 3 based ceramic raw powder is compacted and is subjected to pressureless-sintering, to thus obtain a sintered body containing the SiC whiskers and the sintering auxiliary agent is obtained. Here, prior to the description of the function of a nitrogen containing inert gas atmosphere during sintering, the functions of SiC whiskers and sintering auxiliary agent, which are related to the function of the above atmospheric gas, will be first described. SiC whiskers are effective to improve the toughness of an Al 2 O 3 based ceramic material by the function of pull-out/bridging/crack-deflection. When the content is less than 5 wt%, the effect of improving the toughness is insufficient. When it is more than 30 wt%, the densification of the ceramic material becomes difficult, and thereby the strength thereof is reduced. Accordingly, the content is required to be in the range of from 5 to 30 wt%. In pressureless-sintering a SiC whiskers containing Al 2 O 3 based ceramic material, the selection of a sintering auxiliary agent is an important factor. The sintering auxiliary agent acts to generate a liquid phase during sintering, which accelerates the re-arrangement of entangled SiC whiskers, thereby increasing the density of a sintered body (improving the densification). As the sintering auxiliary agent, oxides of Mg and others are suitable. Either of these sintering auxiliary agents is converted in a liquid phase under a temperature at which SiC whiskers are deteriorated. It does not react with SiC whiskers so much, and is excellent in wettability with SiC whiskers, thus effectively improving the density of a sintered body. When the content of the sintering auxiliary agent is less than 3 wt%, the above effect is insufficient. When it is more than 30 wt%, the amount of Al 2 O 3 is relatively reduced and the excellent characteristics inherent to Al 2 O 3 cannot be obtained. Accordingly, the content of oxides of Mg and others as the sintering auxiliary agent is required to be in the range of from 3 to 14 wt%. The function of a nitrogen containing inert gas atmosphere during sintering will be described below. In the case where a SiC whiskers containing Al 2 O 3 based ceramic material is sintered in vacuum or an Ar gas atmosphere, the sintering auxiliary agent (converted in liquid phase) during sintering is significantly evaporated. This harms the function of the sintering auxiliary agent (the function of accelerating the rearrangement of the entangled SiC whiskers). As a result, a sintered body cannot be densified. On the contrary, in the case where the sintering is performed in a nitrogen containing inert gas atmosphere, the sintering auxiliary agent is prevented from being evaporated, and acts to accelerate the densification, thereby sufficiently densifying the sintered body. The sintered body thus obtained contains nitrogen in an amount of 0.2 wt% or more. The reason for this is unclear, but it may be considered as follows: nitrogen is contained in the sintering auxiliary agent converted in liquid phase during sintering and thereby the evaporation temperature of the liquid phase is increased. However, even in the case where the sintering is performed in such nitrogen containing inert gas atmosphere, when the sintering temperature exceeds 1900°C, the sintering auxiliary agent is significantly evaporated, thus failing to obtain a sufficiently densified sintered body. On the other hand, when it is less than 1500°C, the sufficient liquid phase of the sintering auxiliary agent is not generated, so that the sintering auxiliary agent cannot be sufficiently acted, thus failing to obtain a sufficiently densified sintered body. Accordingly, the sintering temperature is required to be in the range of from 1500 to 1900°C. The nitrogen containing inert gas atmosphere means a nitrogen gas atmosphere or a mixed gas atmosphere of nitrogen gas and an inert gas other than nitrogen gas (for example, argon gas). The pressure of the nitrogen containing inert gas is normal pressure (1 atm) because the sintering is performed at normal pressure. The present invention is accomplished on the basis of the above-described knowledge. A method of manufacturing an Al 2 O 3 based ceramic material according to the present invention, comprises the steps of: compacting a mixed powder containing: 5-30 wt% of SiC whiskers, and 3-14 wt% of a sintering auxiliary agent composed of one kind or more of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements, the balance being an Al 2 O 3 based ceramic raw powder; and pressureless-sintering the green compact at a temperature between 1500 and 1900°C in a nitrogen containing inert gas atmosphere. This makes it possible to manufacture the SiC whiskers containing Al 2 O 3 based ceramic material composed of a sintered body being sufficiently densified and having a sufficient strength and toughness, basically using pressureless-sintering capable of forming a product having a complex shape and a large side. The Al 2 O 3 based ceramic material according to the present invention comprises an Al 2 O 3 sintered body, the Al 2 O 3 sintered body containing: 5-30 wt% of SiC whiskers; 3-14 wt% of a sintering auxiliary agent composed of one kind or more of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements; and 0.2 wt% or more of nitrogen. Accordingly, the Al 2 O 3 based ceramic material is composed of an Al 2 O 3 sintered body being sufficiently densified and having a sufficient strength and toughness, which can be obtained basically using pressureless-sintering. The above mixed powder may be added with 0.5-10 wt% of one carbide, nitride or boride of a transition element selected from groups IVa, Va, and VIa of the periodic table. In this case, part of the compounds is entrapped in crystal grains of Al 2 O 3 , and dislocations are generated in the crystal grains of Al 2 O 3 due to the difference in the thermal expansion coefficient between the compounds and Al 2 O 3 upon cooling after pressureless-sintering. As a result, there can be obtained the Al 2 O 3 based ceramic material being further excellent in the strength and toughness. As for the content of the above compounds, when it is less than 0.5 wt%, the effect of improving the strength and toughness cannot be achieved; while when it is more than 40 wt%, the amount of Al 2 O 3 is relatively reduced and the excellent characteristics inherent to Al 2 O 3 cannot be achieved. In the above Al 2 O 3 based ceramic material containing the compounds of one of carbide, nitride or boride of a transition element selected from groups IVa, Va and VIa of the periodic table, the content of nitrogen other than the nitrogen in any nitride of said transition elements is 0.2 wt% or more. Namely, in the case where the above compounds contain the nitride (the compound of the above transition element and N), the content of nitrogen other than N constituting the nitride is 0.2 wt% or more. On the other hand, in the case where the compounds do not contain the nitride, the content of nitrogen in the Al 2 O 3 based sintered body is 0.2 wt% or more, just as in the Al 2 O 3 based ceramic material not containing the above compound. As described above, the Al 2 O 3 based ceramic material of the present invention is composed of the Al 2 O 3 sintered body being sufficiently densified and having a sufficient strength and toughness. However, by specifying the porosity of the sintered body to be in the range of 1% or less, the strength and toughness of the sintered body can be further enhanced. Additionally, in the sintered body having a porosity of more than 1%, the pores act as the starting points of cracking resulting in the reduced strength, as compared with the sintered body having a porosity of 1% or less. The sintered body having a porosity of 1% or less can be obtained by hot-isostatic-pressing (HIP) the sintered body obtained by pressureless-sintering. In particular, the sintered body having a porosity of 5% or less after pressureless-sintering can be certainly changed into that having a porosity of 1% or less by HIP. At this time, a nitrogen containing inert gas atmosphere is effective in removing pores (easily densifying the sintered body) and reducing the porosity more than in air or in an Ar gas atmosphere, for the same reason in the pressureless-sintering. As for the temperature of HIP, when it is less than 1200°C, the effect of removing pores is insufficient; while when it is more than 1700°C, the growth of grains of Al 2 O 3 reduces the strength and toughness. Accordingly, the temperature of HIP is preferably in the range of from 1200 to 1700°C. Therefore, it is desirable to reduce the sintered body after pressureless-sintering to be 1% or less by applying HIP to the sintered body in an inert gas atmosphere at a temperature between 1200 and 1700°C. In addition, when the pressure of the nitrogen containing inert gas atmosphere is less than 500 atm, pores are difficult to be removed; while it is more than 2000 atm, unnecessary residual stress tends to remain in the ceramic material. Accordingly, the pressure of the nitrogen containing inert gas atmosphere is preferably in the range of from 500 to 2000 atm. The Al 2 O 3 based ceramic material of the present invention contains 3-14 wt% of the sintering auxiliary agent composed of one kind or more of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements. In particular, by use of the sintering auxiliary agent which contains 0.2-10 wt% of MgO, 0.2-20 wt% of ZrO 2 and 1.0-20 wt% of Y 2 O 3 such that the total content of these oxides is 14 wt% or less, even an Al 2 O 3 based ceramic material containing SiC whiskers in a relatively large amount (for example, 15 wt% or more) can be densified and increased in hardness. The reason for this is that the use of the MgO, ZrO 2 and Y 2 O 3 in the above amounts accelerates the sintering and suppresses the growth of grains, thus sufficiently keeping the hardness. In addition, when the total content of the sintering auxiliary agent is less than the above value, the sintering accelerating effect is insufficient; when it is more than the above value, the ratio of the sintering auxiliary agent in the ceramic material becomes so large that the hardness inherent to Al 2 O 3 cannot be achieved. To enhance the above effect, the total content of MgO, ZrO 2 and Y 2 O 3 is preferably in the range of from 10 to 15 wt%, and more preferably, in the range of from 8 to 12 wt%. Incidentally, the particle size of Al 2 O 3 and the share of the SiC whisker in the matrix of an Al 2 O 3 sintered body exert effects on the strength and toughness of the Al 2 O 3 based ceramic material of the present invention. When the particle size of Al 2 O 3 is more than 5 µm, the particles act as the starting points of fracture resulting in the reduced strength. Accordingly, to suppress the reduction of the strength depending on the particle size, the particle size of Al 2 O 3 in the Al 2 O 3 sintered body is preferably in the range of 5 µm or less. The Al 2 O 3 based ceramic material having such particle size can be obtained by suitably controlling the manufacturing conditions using an Al 2 O 3 based ceramic raw powder having a particle size of 5 µm or less. As for the shape of SiC whiskers, the toughness and strength of an Al 2 O 3 based ceramic material can be obtained by setting the average aspect ratio to be 3 or more, and the diameters to be in the range of from 0.4 to 1.5 µm. Here, SiC whiskers function as crack-bridging in an Al 2 O 3 based ceramic material and obstruct the propagation of the cracks. The function is small when the diameters of SiC whiskers are less than 0.4 µm, and it becomes larger as the diameters are made larger. However, when the diameters are more than 1.5 µm, the SiC whiskers themselves act as the starting points of fracture, and thereby the toughness is improved but the strength is reduced. Moreover, when the aspect ratio is less than 3, such function is small. In addition, to further enhance the strength and the toughness, preferably, the aspect of SiC whiskers is 4 or more, and the diameters of SiC whiskers are in the range of from 0.6 to 1.0 µm. According to the present invention, as described above, the content of SiC whiskers must be in the range of from 5 to 30 wt%. However, when it is more than 10 wt%, the contraction during pressureless-sintering is obstructed by the network structure formed by the mutual entanglement of SiC whiskers, and the sintering ability tends to be reduced. In this regard, the content of SiC whiskers may be in the range of 10 wt% or less to prevent the reduction of the sintering ability. However, at this time there occurs another problem in restricting the improvement of the toughness and the strength. For this reason, it is desirable to prevent the reduction of the sintering ability even in the case where the content of SiC whiskers exceeds 10 wt%. The inventors have studied the means for coping with this inconvenience. As a result, it was found that by setting the product of the average aspect ratio of SiC whiskers and the content of SiC whiskers in an Al 2 O 3 sintered body to be 400 or less, the sintering ability is not reduced even in the case where the content of SiC whiskers is more than 10 wt%. In other words, by setting the above product to be 400 or less, the content of SiC whiskers can be set to be in the range of 10 to 30 wt% without any reduction of the sintering ability, thus further improving the toughness and the strength of the sintered body. When the above product is more than 400, the mutual entanglement of SiC whiskers is made larger, and the repulsion thereof lowers the effect of preventing the reduction of the sintering ability. To stabilize the porosity of the Al 2 O 3 sintered body after pressureless-sintering to be 5% or less, the product is preferably less than 300. The Al 2 O 3 based ceramic material in which the average aspect ratio of SiC whiskers is 3 or more can be manufactured by suitably controlling the mixing condition of a mixed powder. When the mixing is performed by a ball mill, the mixing time 〈t〉 is set to be in the range of from 2 to 200 hr and to satisfy the equation of t≥2c-20 (c= content of SiC whiskers, t= hours). When the mixing time 〈t〉 is more than 200 hr, the average aspect ratio becomes a value less than 3 because of breakage of SiC whiskers. When the mixing time 〈t〉 is less than (2c-20), the aspect ratio becomes excessively large, and poor sintering of the mixed powder occurs. When the mixing time 〈t〉 is less than 2 hr, it is difficult to obtain uniform mixing, which lowers the homogeneity of an Al 2 O 3 base ceramic material, resulting in the reduced strength. To obtain the excellent strength, the mixing time is preferably in the range of from 4 to 100 hr. When the mixing of the above mixed powder is performed by a wet mixing method, the obtained mixed powder is dried before compacting. This drying has been made by a tray drying method where the mixed powder is put in a metal tray and stationarily dried for several hours. However, in this drying method, even when the powder is uniformly mixed at the time of mixing, SiC whiskers are aggregated during the stationary drying and are finally made in the non-uniform state, thereby significantly reducing the strength and toughness. As compared with this tray drying, the spray drying can rapidly dry the mixed powder before the generation of the aggregation of SiC whiskers. Accordingly, in the obtained Al 2 O 3 based ceramic material, the homogeneity of SiC whiskers is excellent, and thereby the strength and toughness are enhanced. In general, a ceramic material is compacted at a compacting pressure between 98 and 196 MPa (1 and 2 ton/cm 2 ). However, for the mixed powder of the present invention, the compacting pressure is preferably in the range of 294 MPa (3.0 ton/cm 2 ) or more. This is based on the knowledge obtained in examining the relationship between the compacting pressure and the sintered density. Namely, when a mixed powder containing short fibers such as SiC whiskers is compacted, the usual compacting pressure (98 to 196 MPa) (1 to 2 ton/cm 2 ) is difficult to increase the green density by the effect of SiC whiskers, and a high compacting pressure (294 MPa or more) (3.0 ton/cm 2 or more) is successful to improve the green density. Specifically, the relative density (green density/theoretical density of sintered body) is 50% or less for the usual compacting pressure; but it becomes 53% or more for the compacting pressure of 294 MPa (3.0 ton/cm 2 ) or more. The improvement of the green density means the increase in the contacts between powders and/or between SiC whiskers and powders, which accelerates the densification during sintering, resulting in the improved strength and toughness. At this time, as the compacting pressure is higher, the strength and toughness become higher. In this regard, the compacting pressure is desirable to be 392 MPa (4.0 ton/cm 2 ). However, from the viewpoint of the prevention of cracks, the compacting pressure is desirable to be 980 MPa (10 ton/cm 2 ) or less. In addition, the compacting of a mixed powder of the present invention may be performed by die-pressing, cold-isostatic-pressing or the like. The present invention will be more clearly understood by way of the following examples: Example 1 An Al 2 O 3 powder (Al 2 O 3 based ceramic material as a base material), SiC whiskers and a sintering auxiliary agent were wet-mixed, followed by spray-drying, and compacted by die-pressing, thus obtaining a green compact. The green compact was subjected to pressureless-sintering in a nitrogen gas (N 2 ) atmosphere at a temperature between 1500 and 1900°C, thus obtaining a sintered body (hereinafter, referred to as "pressureless sintered body"). Some of these pressureless sintered bodies thus obtained were subjected to hot-isostatic-pressing (HIP). This sintered body after being subjected to HIP is referred to hereinafter, as "HIP material". Here, the content of SiC whiskers, the kind and content of a sintering auxiliary agent, pressureless-sintering conditions (temperature, pressure), HIP conditions (temperature, time, pressure) are changed as shown in Table 1. The pressureless sintered bodies and HIP materials thus obtained were measured for porosity, bending strength and fracture toughness, and further analyzed for the content of nitrogen. The porosity was obtained on the basis of the density measured by an Archimedes' method. The bending strength was measured by a three-point bending test. The fracture toughness was measured by an IF (Indentation Fracture) method in accordance with JIS. The content of nitrogen was analyzed by an inert gas melting method. The measured results are shown in Table 2. In the comparative examples (which do not satisfy the requirements of the present invention) the porosity was generally larger and the bending strength and the fracture toughness were small. Contrary, to these examples, in the inventive examples (which satisfy the requirements of the present invention), the porosity was generally small, and the bending strength and the fracture toughness were high. In addition, in Comparative Example No. a-2 (HIP material), the porosity was smaller and the bending strength was higher than those in Inventive Example No. A-5 (pressureless sintered body). This is due to the effect of hot-isostatic-pressing (HIP). However, as for the fracture toughness, Comparative Example No. a-2 is lower than Inventive Example No. A-5. Example 2 Pressureless sintered bodies and HIP materials were obtained in the same manner as in Example 1, except that the content of SiC whiskers, kind and content of a sintering auxiliary agent, pressureless-sintering conditions and HIP conditions were changed as shown in Table 3. The characteristics of these pressureless sintered bodies and the HIP materials were measured in the same manner as in Example 1. The measured results are shown in Table 4. Compared with the comparative examples, the porosity in the inventive examples, was generally small and the bending strength and fracture toughness were high. Example 3 An Al 2 O 3 powder, SiC whiskers, a sintering auxiliary agent, powders of TiC or VC (compound of Ti or V as a transition element and C) were wet-mixed, followed by spray-drying and compacting, and were subjected to pressureless-sintering, thus obtaining a sintered body. Some of these sintered bodies thus obtained were subjected to HIP, thus obtaining HIP materials. Here, the content of SiC whiskers, kind and content of a sintering auxiliary agent, content of TiC or VC, pressureless-sintering conditions, and HIP conditions are changed as shown in Table 5. The characteristics of these pressureless sintered bodies and HIP materials were measured in the same manner as in Example 1. The measured results are shown in Table 6. Compared with the comparative examples, the porosity in the inventive examples was generally small and the bending strength and the fracture toughness were high. Example 4 An Al 2 O 3 powder, SiC whiskers and a sintering auxiliary agent in the composition shown in Table 7 were wet-mixed and compacted. The green compact was subjected to pressureless-sintering in nitrogen at 1500 to 1900°C, which was further subjected to HIP in nitrogen. The hardness was measured using a micro Vickers hardness tester in accordance with JIS. The measured results are shown in Table 8. As is apparent from Table 8, the inventive examples are excellent in mechanical properties and have high hardness. Comparative Example d-4, in which the kind of sintering auxiliary agent is the same as in the inventive example but the content thereof is larger, is low in hardness. Comparative Example d-5, in which the content of the sintering auxiliary agent is smaller than that of the inventive example, is poor in the densification and the mechanical properties. Example 5 The Al 2 O 3 sintered body of Inventive Example No. D-2 in Table 7 of Example 4 has a bending strength of 940.8 MPa (96 kg/mm 2 ) and a fracture toughness is 4.9 MPam 1/2 (see Table 8). The particle size of the matrix (Al 2 O 3 ) of this sintered body was 1.2 µm. In this example, an Al 2 O 3 sintered body having the same composition as that of Inventive Example D-2 was manufactured in the same manner as in Example 4 except for using an Al 2 O 3 based ceramic raw powder having a particle size larger than that of Inventive Example No. D-2. The sintered body thus obtained was measured for the particle size of the matrix and the bending strength and the fracture toughness. As a result, in this sintered body, the particle size was 7.5 µm, the bending strength was 34 kg/mm 2 , and the fracture toughness was 5.7 MPam 1/2 . SiC whisker : wt % Compound: wt% Sintering auxiliary agent: wt% Compacting method Sintering condition HIP condition Inventive example A 1 10 - MgO:2 Die-pressing 1750°C × 1 hr, in N 2 1500°C × 1 hr, 1500 atm, in N 2 Y 2 O 3 :4 2 12.5 - ZrO 2 :4 " 1800°C × 1 hr, in N 2 " MgO:8 3 15 - TiO 2 :4 " 1850°C × 1 hr, in N 2 " ZrO 2 :6 4 20 - MgO:2 " 1750°C × 1 hr, in N 2 " ZrO 2 :4 Y 2 O 3 :8 5 10 - SiO 2 : 5 " 1700°C × 1 hr in N 2 -(:No HIP) Y 2 O 3 :3 Comparative example 〈a〉 1 15 - SiO 2 :8 " 1800°C × 1 hr, in Ar 1500°C × 1 hr, in N 2 Y 2 O 3 :4 2 3 - MgO:2 " 1750°C × 1 hr, in N 2 " Y 2 O 3 :4 3 35 - MgO:2 " 1800°C × 1 hr, in N 2 " Y 2 O 3 :4 4 15 - MgO:12 " " " Y 2 O 3 :24 5 15 - SiO 2 :0.5 " " " Y 2 O 3 :1.5 Amount of N 2 :% Porosity Bending strength: (kg/mm 2 ) MPa Fracture toughness: MPam 1/2 Inventive example A 1 0.6 0.2 (80) 784 4.3 2 0.9 0.2 (84) 823 4.9 3 1.2 0.3 (80) 784 5.1 4 1.2 0.6 (80) 784 5.5 5 0.5 2.0 (65) 637 4.4 Comparative example 〈a〉 1 0.09 2.9 (42) 412 3.2 2 0.8 0.08 (71) 696 3.1 3 0.9 7.8 (31) 304 Measurement; impossible 4 2.1 1.2 (36) 353 2.9 5 0.2 10.5 (25) 245 Measurement; impossible SiC whisker :wt% Compound: wt% Sintering auxiliary agent: wt% Compacting method Sintering condition HIP condition Inventive example B 1 10 - MgO:2 Die-pressing 1750°C × 1 hr, in N 2 1500°C × 1 hr, 1500 atm, in N 2 Y 2 O 3 :4 2 12.5 - ZrO 2 :4 " 1800°C × 1 hr, in N 2 " MgO:8 3 15 - SiO 2 :8 Extrusion " " Y 2 O 3 :4 4 12.5 TiC:4 " Die-pressing " " 5 15 - TiO 2 :4 " 1850°C × 1 hr, in N 2 " ZrO 2 :6 6 20 - MgO:2 " 1750°C × 1 hr, in N 2 " ZrO 2 :4 Y 2 O 3 :8 7 10 - SiO 2 :5 " 1700°C × 1 hr in N 2 -(: No HIP) Y 2 O 3 :3 8 12.5 - SiO 2 : 8 Casting 1800°C × 1 hr in N 2 1500°C × 1 hr, 1500 atm, in N 2 Y 2 O 3 :4 Comparative example 〈b〉 1 3 - MgO: 2 Die-pressing 1750°C × 1 hr, in Ar 1500°C × 1 hr, in N 2 Y 2 O 3 :4 2 35 - " " 1800°C × 1 hr, in N 2 " 3 15 - MgO:12 " " " Y 2 O 3 :24 4 15 - MgO:0.5 " " " Y 2 O 3 :1.5 5 15 - SiO 2 :8 " 1800°C × 1 hr, in Ar 1500°C × 1 hr, in Ar Y 2 O 3 :4 6 15 - " " 1400°C × 1 hr, in N 2 1500°C × 1 hr, in N 2 7 15 - " " 2000°C × 1 hr, in N 2 " Amount of N 2 :% Porosity Bending strength: (kg/mm 2 ) MPa Fracture toughness: MPam 1/2 Inventive example 1 0.6 0.2 (80) 784 4.3 2 0.9 0.2 (84) 823 4.9 3 1.2 0.1 (95) 931 5.1 4 1.3 0.1 (89) 872 5.0 5 1.2 0.3 (80) 784 5.0 6 1.2 0.6 (80) 784 5.5 7 0.5 2.0 (65) 637 4.4 8 0.8 0.1 (83) 813 5.2 Comparative example 〈b〉 1 0.8 0.08 (71) 696 3.1 2 0.9 7.8 (31) 304 Measurement; impossible 3 2.1 1.2 (36) 353 2.9 4 0.21 10.5 (25) 245 Measurement; impossible 5 0.09 2.9 (42) 412 3.2 6 2.0 15.8 (22) 216 Measurement; impossible 7 2.4 6.5 (30) 294 3.1 SiC whisker : wt% Compound: wt% Sintering auxiliary agent:wt% Compacting method Sintering condition HIP condition Inventive example C 1 10 TiC:4 SiO 2 :5 Die-pressing 1750°C × 1 hr, in N 2 1500°C × 1 hr, 1500 atm, in N 2 Y 2 O 3 :3 2 15 TiC:8 MgO:4 " " " Y 2 O 3 :8 3 15 VC:4 MgO:4 " " " ZrO 2 :8 4 20 TiC:4 MgO:4 " " " Y 2 O 3 :8 5 10 TiC:4 SiO 2 :5 " " -(: No HIP) Y 2 O 3 :3 Comparative example 〈c〉 1 15 TiC:8 MgO:4 " 1750°C × 1 hr, in Ar 1500°C × 1 hr, 1500 atm, in Ar Y 2 O 3 :8 2 3 TiC:4 SiO 2 :5 " 1750°C × 1 hr, in N 2 1500°C × 1 hr, 1500 atm, in N 2 Y 2 O 3 :3 3 35 TiC:4 SiO 2 :5 " " " Y 2 O 3 :3 4 10 TiC:4 SiO 2 :0.5 " " " Y 2 O 3 :1.5 5 10 TiC:4 SiO 2 :12 " " " Y 2 O 3 :24 Amount of N 2 :% Porosity Bending strength: (kg/mm 2 ) MPa Fracture toughness: MPam 1/2 Inventive example C 1 1.0 0.1 (92) 902 4.1 2 1.2 0.2 (84) 823 4.9 3 1.4 0.2 (82) 804 5.0 4 1.0 0.5 (80) 784 5.5 5 0.5 3.5 (75) 735 4.8 Comparative example 〈c〉 1 0.08 3.4 (60) 588 4.0 2 0.9 0.1 (82) 804 3.1 3 0.9 11.1 (28) 274 Measurement; impossible 4 1.6 8.6 (30) 294 Measurement; impossible 5 1.2 0.5 (42) 412 2.8 SiC whisker : wt% Compound: wt % Sintering auxiliary agent: wt% Sintering condition HIP condition Inventive example D 1 15 - MgO:1, 1750°C × 1 hr 1450°C × 1 hr ZrO 2 : 2, Y 2 O 3 :4 2 15 TiC:4 MgO:1, 1800°C × 1 " ZrO 2 :2, Y 2 O 3 :4 hr 3 15 TiC: 8 MgO:1.4, 1820°C × 1 " ZrO 2 :3, Y 2 O 3 :6 hr 4 20 - MgO:1.8, 1750°C × 1 hr " ZrO 2 :4, Y 2 O 3 :8 5 15 VC: 2 MgO:1, 1750°C × 1 hr " WC:4 ZrO 2 :2,Y 2 O 3 :4 6 10 TiN:4 MgO:1.8, 1750°C × 1 hr " ZrO 2 :4, Y 2 O 3 :8 7 12.5 TiB 2 : 10 MgO: 1.8, 1770°C × 1 hr " ZrO 2 :4, Y 2 O 3 :8 Comparative example 〈d〉 4 15 - MgO:5, ZrO 2 :10, Y 2 O 3 :15 1750°C × 1 hr " 5 15 - MgO:0.1, ZrO 2 : 0.1, Y 2 O 3 :0.5 1800°C X 1 hr " Amount of N 2 :% Porosity: % Bending strength: (kg/mm 2 ) MPa Fracture toughness: MPam 1/2 Hardness Hv Inventive example D 1 0.4 0.0 (100) 980 5.4 1820 2 0.8 0.2 (96) 941 4.9 1900 3 1.0 0.1 (110) 1078 5.4 2100 4 0.7 0.3 (86) 843 5.1 1880 5 0.5 0.0 (100) 980 5.6 2050 6 1.7 0.2 (88) 862 4.8 1900 7 0.6 0.3 (85) 833 4.9 1950 Comparative example 〈d〉 4 1.2 0.2 (70) 686 4.6 1600 5 0.3 7.2 (46) 451 4.6 1520 SiC whisker Mixing time: Hr Porosity : % Bending strength: (kg/mm 2 ) MPa Fracture toughness: MPam 1/2 Diameter: µm Aspect ratio Content :wt% Inventive example D-2 0.8 15 15 20 0.2 (96) 941 4.9 Comparative example E 1 0.2 15 15 20 0.2 (83) 813 3.0 2 2.0 15 15 20 0.2 (62) 608 5.5 3 0.8 2 15 240 0.2 (68) 666 2.7 4 0.8 50 15 5 8.8 (35) 343 Measurement; impossible 5 0.8 30 25 20 21.1 (23) 225 Measurement; impossible 6 0.8 60 5 1 0.1 (43) 421 3.2 Example 6 An Al 2 O 3 sintered body having the same composition as that of Inventive Example No. D-2 was manufactured in the same manner as in Example 4, except that the diameter, average aspect ratio, and content 〈c〉 of SiC whiskers; raw powder mixing time 〈t〉 in a ball mill; and a porosity were changed as shown in Table 9. The sintered body was measured for bending strength and fracture toughness. As a result, as shown in Table 9, in the Al 2 O 3 sintered body of Inventive Example No. D-2, the diameter of SiC whiskers is 0.8 µm; the aspect ratio is 15; the content 〈c〉 is 15%; the mixing time 〈t〉 is 20 hr; the porosity is 0.2; the bending strength is 96 kg/mm 2 ; and the fracture toughness is 4.9 MPam ½ . On the contrary, of the Comparative Examples Nos. E1 to E6, in Comparative Example No. E1 the diameter of SiC whiskers is less than 0.4 µm, so that the bending strength is substantially similar to that of Inventive Example No. D-2 but the fracture toughness is lower. In Comparative Example No. E2, the diameter of SiC whiskers is larger than 1.5 µm, so that the bending strength is lower though the fracture toughness is similar to that of Inventive Example No. D-2. In Comparative Example No. E3, the mixing time 〈t〉 is more than 200 hr and therefore the aspect ratio is less than 3, so that the bending strength and the fracture toughness are lower. In Comparative Example Nos. E4 and E5, the mixing time 〈t〉 is less than (2c-20) and therefore the aspect ratio is large, so that the product of the aspect ratio and the content 〈c〉 of SiC whiskers is more than 400, as a result of which the densification is poor and the porosity is large. In Comparative Example No. E6, the mixing time 〈t〉 is less than 2 hr, so that the structure is not homogeneous, resulting the reduced strength. Example 7 In the Al 2 O 3 sintered body of Inventive Example No. D-2, wherein the drying of the mixed powder after wet mixing is performed by spray drying, the bending strength is 941 MPa (96 kg/mm 2 ) and the fracture toughness is 4.9 MPam 1/2 . An Al 2 O 3 sintered body having the same composition as that of Inventive Example No. D-2 was obtained in the same manner as in Example 4 except that the spray drying was replaced by the tray drying. In the sintered body thus obtained, the bending strength was 363 MPa (37 kg/mm 2 ) and the fracture toughness was 4.2 MPam 1/2 . Example 8 Using a mixed powder having the same composition as that of Inventive Example No. D-2, the effect of the compacting pressure in die-pressing or cold-isostatic-pressing (hereinafter, referred to as CIP) the mixed powder exerted on the densities of a green compact, sintered body and HIP material was examined. The results are shown in Fig. 1 (for die pressing) and Fig. 2 (for CIP). As the compacting pressure is increased, the green density becomes high and thereby the sintered density becomes high. The density of the HIP material reaches about 100% in the relative density when the compact pressure is 294 MPa (3.0 ton/cm 2 ) or more. Example 9 A mixed powder containing 15 wt% of SiC whiskers, a sintering auxiliary agent composed of 0.1 wt% of MgO, 2 wt% of ZrO 2 and 4 wt% of Y 2 O 3 , and an Al 2 O 3 based ceramic raw powder as a base material, or a compound shown in Table 10 was compacted, and then subjected to pressureless-sintering and HIP under the conditions shown in Table 10. The Al 2 O 3 based ceramic material thus obtained was measured for the density, bending strength and fracture toughness. The results are shown in Table 10. In Inventive Example Nos. F1 to F6, which are compacted at a compacting pressure of 294 MPa (3 ton/cm 2 ), the bending strength and fracture toughness are high. On the contrary, in Comparative Example Nos. G1 to G3, which are compacted at a compacting pressure of less than 294 MPa (3 ton/cm 2 ), the bending strength and fracture toughness are low.

Claims: An Al 2 O 3 ceramic material comprising an Al 2 O 3 sintered body, said Al 2 O 3 sintered body comprising: 5-30 wt% of SiC whiskers, 3-14 wt% of a sintering auxiliary agent selected from one or more oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements, and 0.2 wt% or more of nitrogen, obtainable by a method comprising the steps of: compacting a mixed powder comprising: 5-30 wt% of SiC whiskers, and 3-14 wt% of said sintering auxiliary agent, the balance being an Al 2 O 3 ceramic raw powder, and pressureless-sintering the green compact at a temperature between 1500 and 1900°C in a nitrogen containing inert gas atmosphere. An Al 2 O 3 ceramic material according to claim 1 which comprises: 0.5-10 wt% of at least one carbide, nitride or boride of a transition element selected from groups IVa, Va, and VIa of the periodic table, said 0.2 wt% or more of nitrogen excludes any nitride of said transition element, obtainable by said method wherein said mixed powder additionally comprises 0.5-10 wt% of at least one carbide, nitride or boride of said transition element. An Al 2 O 3 ceramic material according to claim 1 or claim 2 wherein said Al 2 O 3 sintered body has a porosity of 1% or less and which is obtainable by said method which further comprises the step of hot-isostatic-pressing said sintered body obtained by the pressureless-sintering at a temperature between 1200 and 1700°C in a nitrogen containing inert gas atmosphere, thereby reducing the porosity of said sintered body to 1% or less. An Al 2 O 3 based ceramic material according to any of claims 1 to 3, wherein said sintering auxiliary agent contains 0.2-10 wt% of MgO, 0.2-20 wt% of ZrO 2 and 1.0-20 wt% of Y 2 O 3 such that the total content of these oxides is 14 wt% or less. An Al 2 O 3 based ceramic material according to any of claims 1 to 4 wherein the particle size of Al 2 O 3 in said Al 2 O 3 sintered body is 5 µm or less. An Al 2 O 3 ceramic material according to any of claims 1 to 5 wherein the average aspect ratio of said SiC whiskers is 3 or more, and the diameters of said SiC whiskers are in the range of from 0.4 to 1.5 µm. An Al 2 O 3 ceramic material according to any of claims 1 to 6 wherein the product of the average aspect ratio of said SiC whiskers and the content of said SiC whiskers in said Al 2 O 3 sintered body is 400 or less. A method of manufacturing Al 2 O 3 ceramic material according to claim 1 comprising the steps of: compacting a mixed powder containing: 5-30 wt% of SiC whiskers, 3-14 wt% of a sintering auxiliary agent selected from one or more oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements, the balance being an Al 2 O 3 ceramic raw powder, and pressureless-sintering the green compact at a temperature between 1500 and 1900°C in a nitrogen containing inert gas atmosphere. A method of manufacturing Al 2 O 3 ceramic material according to claim 2 comprising the steps of: compacting a mixed powder containing: 5-30 wt% of SiC whiskers, 0.5-10 wt% of at least one carbide, nitride or boride of a transition element selected from groups IVa, Va, and VIa of the periodic table, 3-14 wt% of a sintering auxiliary agent selected from one kind or more oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements, the balance being an Al 2 O 3 ceramic raw powder, and being an Al 2 O 3 ceramic raw powder, and pressureless-sintering the green compact at a temperature between 1500 and 1900°C in a nitrogen containing inert gas atmosphere. A method of manufacturing Al 2 O 3 ceramic material according to claim 8 or claim 9, which further comprises the step of hot-isostatic-pressing said sintered body obtained by the pressureless-sintering at a temperature between 1200 and 1700°C in a nitrogen containing inert gas atmosphere, thereby reducing the porosity of said sintered body to 1% or less. A method of manufacturing an Al 2 O 3 ceramic material according to any of claims 8 to 10 wherein the mixing to obtain said mixed powder is performed using a ball mill, and the mixing time is set to be in a range to satisfy the equation t≥2c-20 where c≥5 and c≤30 and t≥2 and t≤200, wherein 〈c〉 is the content (wt%) of SiC whiskers in said mixed powder, and 〈t〉 is the mixing time (hr). A method of manufacturing an Al 2 O 3 ceramic material according to any of claims 8 to 11 wherein the mixing to obtain said mixed powder is performed by a wet mixing method, and the drying of said mixed powder after mixing and before compacting is performed by a spray-drying method. A method of manufacturing an Al 2 O 3 ceramic material according to any of claims 8 to 12 wherein the compacting pressure for said mixed powder is in the range of from 294 MPa(3.0 ton/cm 2 ) or more.

Inventors: KANAMARU MORIYOSHI, TATSUNO TSUNEO, TSUCHIDA TAKEHIRO, KANAMARU, MORIYOSHI, TATSUNO, TSUNEO, TSUCHIDA, TAKEHIRO, KANAMARU, MORIYOSHI, C/O KOBE CORPORATE RES. LAB., TATSUNO, TSUNEO, C/O KOBE CORPORATE RES. LAB., TSUCHIDA, TAKEHIRO, C/O KOBE CORPORATE RES. LAB.
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1994
Main classification: C04B35
IPC codes: C04B35, B23P15, B23B27